trigger ATaskUser_BfIn_Tr on A_Task_User__c (before insert) {
List<A_Task_User__c> aTaskUsr_List=[Select User__c, Name, Id, Announcement_Task__c From A_Task_User__c WHERE (User__c=:trigger.new[0].User__c) AND (Announcement_Task__c=:trigger.new[0].Announcement_Task__c)];
if(aTaskUsr_List.size()>0)
{
trigger.new[0].name.addError('User Name already Exist on this Task');
}

}